[TowerTalk] POLYPHASERS V CUSHCRAFT

[ROBERT A. WANDERER]

     Alfred
     
     While the Cushcraft devices DO work, the PolyPhasers are 
     demonstrably better protection. Both devices, in fact any brand 
     of protector, are only as good as the lightning ground system 
     provided. If you provide me a snail mail address, I shall send to 
     you our catalog and other material which provide an overview as 
     to what constitutes a good lightning ground system. In addition 
     to a proper lightning ground and a single point ground system, we 
     suggest several feet of coax be placed between the PolyPhaser and 
     the equipment. This adds some inductance so that the surge is  
     more likely to see the ground as a lower inductive path to earth. 
     The name of the game is inductance (or, adding in the frequency 
     component, impedance). For that reason, the ground system should 
     utilize solid copper strap. If cable must be used, #2 is the 
     minimum you can "get away with," although we would recommend at 
     least 2/0 and 4/0 is even better (albeit rather difficult to work 
     with even compared to strap). Note that 1.5"-wide copper strap 
     has less inductance per foot than 2/0! Suffice it to say that one 
     ground rod and some #6 wire does not a good lightning ground 
     make!
     
     The Cushcraft gas tube fires at some voltage which has to be high 
     enough to prevent it from being turned on by the RF voltage 
     (known as glow mode). I don't know what value Cushcraft uses, but 
     for argument's sake let's say it's 600 Vdc. Any voltage below 
     that point (and actually somewhat above that value, but let's 
     keep it simple) will get through to your equipment. In addition, 
     the gas tube puts out its own slew voltage (dV/dt) when it fires, 
     and this gets into your equipment too. Granted the time factor is 
     very quick. Note that a strike several miles away may cause a 
     surge of less than this 600 V but greater than what your 
     equipment would like to see to come down the coaxial cable. Even 
     grounding the shield is not enough of a cure because we're 
     talking surge current on the center conductor.
     
     The PolyPhaser is dc-blocked on the center conductor. This means 
     that almost none of the strike energy gets through. Realize that 
     the bulk of the energy is at dc, with a "spike" between 10 kHz 
     and about 1 MHz. Due to the fast rise time (microseconds), there 
     is energy, albeit low in level compared to the dc and 10-1000 
     kHz, into the GHz range. We publish our throughput energy 
     (usually in milli to micro Joules); does Cushcraft (or any of our 
     competitors)? Obviously, this design protects the equipment from 
     the gas tube's slew voltage too.
     
     We also make a model where the shield is dc-blocked as well. This 
     is the best possible protection especially in situations where 
     the quality of the ground may be far less than what's desired. 
     
     Hope this helps!
     
     Sincerely and 73,
     Bob Wanderer AA0CY
     Senior Applications Engineer
     PolyPhaser Corporation

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